Here is another version of inverse iteration method, where if statement works fine.
function [x,iter] = invitr(A, ep, numitr)
%INVITR Inverse iteration
%[x,iter] = invitr(A, ep, numitr) computes an approximation x, smallest
%eigenvector using inverse iteration. initial approximation is vector of ones,
%ep is the tolerance and numitr is the maximum number of iterations.
%If the iteration converged, iter is the number of iterations
%needed to converge. If the iteration did not converge,
%iter contains numitr.
%This program implements Algorithm in
%http://www.netlib.org/utk/people/JackDongarra/etemplates/node96.html
%input : Matrix A, ep and integer numitr
%output : vector x and integer iter
[m,n] = size(A);
if m~=n
disp('matrix A is not square');
return;
end;
y=ones(n,1);
for k = 1 : numitr
iter = k;
v = y/norm(y,2);
y = A\v;
th =v'*y;
if norm(y-th.*v,2) < ep*abs(th)
break;
end;
end;
x = y/th;
end
